Micro-combustor system for the production of electrical energy

ABSTRACT

A system for the production of electrical energy, comprising: a combustion chamber ( 14 ) made of material that is able to withstand high temperatures, an injection device ( 16 ) connected to said combustion chamber ( 14 ) by means of an injection conduit ( 15 ), means ( 17 ) for supplying combustion support substance into the combustion chamber ( 14 ) and means ( 18 ) for the removal of gaseous combustion products, means ( 26 ) for the selective emission of radiation onto the outer surface of the combustion chamber ( 14 ). The combustion chamber ( 14 ) is enclosed in a conversion chamber ( 20 ) within which are maintained sub-atmospheric pressure conditions, so that a substantial part of the heat developed by the combustion reaction is converted into electromagnetic radiation.

This application is the US national phase of international applicationPCT/IB2003/004908 filed 3 Nov. 2003 which designated the U.S. and claimsbenefit of IT TO2002A001083, dated 13 Dec. 2002, the entire content ofwhich is hereby incorporated by reference.

The present invention relates to a micro-combustor system for theproduction of electrical energy.

The present invention is based on the physical principle whereby thermalenergy produced by a combustion is transformed into electromagneticenergy, which in turn is converted into electrical energy, for instanceby means oF photovoltaic cells made of semiconductor material.

The object of the present invention is to provide a micro-combustorsystem for the production of electrical energy with high efficiency ofconversion of the thermal energy into electrical energy.

According to the present invention, this object is achieved by a systemhaving the characteristics set out in the main claim.

The present invention shall now be described in detail with reference tothe accompanying drawings, provided purely by way of non limitingexample, in which:

FIG. 1 is a schematic perspective view of a micro-combustor systemaccording to the present invention, and

FIG. 2 is a perspective sectioned view of a conversion device indicatedby the arrow II in FIG. 1.

With reference to FIG. 1, the number 10 schematically designates amicro-combustor system for the production of electrical energy. Thesystem 10 comprises a plurality of conversion devices 11, electricallyconnected to each other in series or in parallel, each of which isconstructed as described hereafter. The system 10 comprises a pipelineof conduits 12 to supply fuel and combustion supporter to the individualconversion devices 11, a pipeline 13 of exhaust conduits for the removalof gaseous combustion products from the conversion devices 11 and anetwork of electrical connections, for regulating generated power, forthe electrical ignition of the combustors and for transporting thecurrent from the combustor to the load resistance.

With reference to FIG. 2, each conversion device 11 comprises acombustion chamber 14 made of material that is able to withstand hightemperature. Preferably, the combustion chamber has spherical shape andis constituted by such material as to withstand temperatures in theorder of 1500-2000 K.

The combustion chamber is preferably provided with means for theselective emission of electromagnetic radiation, preferably made in theshape of a lining 26 applied onto the outer surface of the combustionchamber 14. The combustion chamber is preferably constituted by amaterial with high heat conductivity (for instance tungsten ormolybdenum), to allow the heat generated by combustion to reach theouter surface 26. At least a part of the inner surface of the combustionchamber 14 is preferably coated with a material with low heatconductivity of the meso-porous or nano-porous type with porosity coatedby catalysing agents, having the function of lowering the combustionactivation temperature and reducing emissions of polluting reactionproducts (for instance nitrogen oxides). The material with low heatconductivity can be interleaved with the material with high heatconductivity in the form of a composite.

The lining 26 preferably has a selective emissivity in a wavelength bandof a few hundredths of nanometres. The lining 26 can for instance beconstituted by a micro-structure obtained directly on the outer surfaceof the combustion chamber, or a thin layer of oxide having a highlyselective spectral emission (oxide of yttrium, thorium, cerium,europium, erbium, terbium, ytterbium or other rare earth).

The combustion chamber 14 communicates with a fuel injection conduit 15,with a conduit 17 for supplying the combustion support and with aconduit 18 for the exhaust of gaseous reaction products. The conduit 15preferably has cylindrical shape with a conical terminal segment, inproximity to the micro-injection system 16, with a section thatincreases outwardly; the purpose of the conical terminal section is toassure that the combustion support substance is aspirated by Venturieffect. The conduits 15, 18 are preferably constituted by ceramicmaterial, or other material with low heat conductivity, to prevent theheat of the combustion chamber to propagate by thermal conduction to theexterior. The outermost part of the exhaust conduit 18 is preferablymetallic to allow exhaust gases to release their residual heat beforeleaving the conversion chamber. The conduit 15 may have an articulatedshape, for instance a spiral or a coil, to prevent the combustionproducts from returning towards the micro-injector. Similarly, theexhaust conduit 18 can have articulated shape to favour the cooling ofthe combustion products. The supply conduit 17 is preferably connectedto the injection conduit 15; alternatively, it can be connected directlyto the combustion chamber. The conduit 17 for supplying the combustionsupporting substance can be eliminated if a mixture of fuel pre-mixedwith liquid or gaseous combustion supporting substance is injected intothe injection conduit 15.

The combustion chamber 14 is closed and it does not exchange gaseousproducts with the exterior except through the conduits 15, 17 and 18.

Each conversion device 11 is provided with a micro-injection device 16preferably constituted by an ink-jet injector, of the “bubble” type orof the piezoelectric type, able to inject drops of fuel or acombustion-support substance mixture of a volume of a few picolitres andwith a frequency which can be controlled by means of a controller (30)from a few kHz to hundreds of kHz. Alternatively, if a gaseous fuel isused, the injection system can be constituted by a miniaturised Bunsenburner. The fuel injected by the injection system 16 penetrates insidethe combustion chamber 14 through the injection conduit 15. Preferably,the gaseous fuel injected by the injection device 16 is selected withinthe group comprising: methane, propane, butane, hydrogen, natural gas orother fuels including the possibility of adding metallic particles tothe fuel.

Each conversion device 11 comprises a hollow structure 19 forming asealed conversion chamber 20, within which is obtained a vacuum or iscontained an inert gas at low pressure. The combustion chamber 14 islocated inside the conversion chamber 20 and the conduits 15, 18 extendthrough the walls of the hollow structure 19. The walls of the hollowstructure 19 defining the conversion chamber 20 can be made of metal, ifa vacuum is obtained in the hollow structure 19, or of ceramic materialcoated with a high reflectance layer, in all other cases.

The hollow structure 19 comprises an elliptical wall 21 and a planarwall 22, so the conversion chamber 20 has the shape of a rotationalsemi-ellipsoid with half-axes A and B. The dimensions of the axes of theconversion chamber 20 may vary from a minimum of 3 to 50 times thediameter of the combustion chamber 14. The combustion chamber 14 ispreferably positioned in the first focus of the elliptical surface. Theinner surface of the elliptical wall 21 is preferably provided with alining 23 having high reflectance over the entire emission spectrum ofthe source of radiation.

The planar wall 22 of the hollow structure 19 bears means for convertingelectromagnetic energy into electrical energy, schematically designatedby the reference number 24. Said conversion means are preferablyconstituted by photovoltaic cells made of semiconductor material,preferably with a band gap in the order of 0.5-0.8 eV in order tomaximise the conversion efficiency by Planck radiation with colourtemperature of 1500-2000 K. In a preferred embodiment, the photovoltaiccell is of the Schottky type and the active junction is constituted bysilicon and aluminium. In the case of the selective electromagneticenergy the material of the cells 24 constituting the conversion means isselected in such a way that the band gap energy is slightly greater thanthe energy of the photons corresponding to the wavelength of maximumemission, in order to maximise the conversion efficiency at thatwavelength.

The exterior face of the conversion means 24 is preferably coated by areflective metal layer. The inner wall of the conversion means 24 can becoated by a layer operating on the electromagnetic radiation as a bandpass filter. Said layer can be a multi-layered dielectric coating, ametallic coating at the percolation state, an anti-reflectionmicro-structure (for instance a grid with sub-wavelength period) or aphotonic crystal.

The conversion means 24 are positioned in correspondence with the planethat is perpendicular to the greater axis of the ellipsoid and passingthrough the centre of the ellipsoid, in such a way that the radiationemitted by the combustion chamber 4 reaches the photovoltaic meansuniformly. Moreover, also by means of the selected geometry, theradiation not absorbed by the conversion means 24 is reflected by thereflecting rear face or by the front surface of the photovoltaic cell 24and falls back onto the combustion chamber 14 where it is absorbed.

The particular geometry of the conversion chamber 20 causes both theradiation emitted by the combustion chamber and reflected by thephotovoltaic chamber 24, and the radiation emitted by the combustionchamber 14 and reflected by the inner walls of the semi-ellipsoid to beconcentrated on the combustion chamber 14. This assures a maximumrecycling of the electromagnetic energy within the conversion chamberand hence a minimisation of fuel consumption and a maximisation ofoverall conversion efficiency. The radiation reflected by the innersurface of the semi-ellipsoid or by the photovoltaic cell 24 isre-absorbed by the lining 26 with the same efficiency with which it isemitted thereby.

The heat developed by the fuel-support substance reaction warms thesurfaces of the combustion chamber and is wholly converted intoelectromagnetic radiation. The dimension of the conduits 15, 18extending within the conversion chamber 20 is such as to minimise thetransfer of thermal energy by conduction between the combustion chamber14 and the hollow structure 19. The radiation emitted inside theconversion chamber 20 impacts on the conversion means 24 which convertelectromagnetic radiation into electric energy. The electrical powergenerated by each conversion device 11 can vary from a few watts to sometens of watts. Each device 11 is provided with electrical contacts (notshown herein) which collect electrical energy produced by thesemiconductor cells 24.

Maintaining a vacuum or sub-atmospheric pressure conditions inside thecombustion chamber 20 allows to reduce the quantity of thermal energydispersed by convection. Consequently, nearly all the heat developed bythe combustion reaction is converted into electromagnetic radiationwhich in turn is converted into electrical energy by the conversionmeans 24. To obtain a vacuum or low pressure conditions within theconversion chamber 20, various known techniques for assemblingcomponents in a vacuum may be used.

1. A portable device for the production of electrical energy, comprisinga matrix of one or more conversion modules, operating in series or inparallel, wherein each of the conversion modules comprises: a combustionchamber having an outer surface defining a substantially spherical shapeand made of material that is able to withstand high temperatures, anexternal lining having a selective radiation emissivity in apredetermined wavelength on the outer surface of the combustion chamber,means for supplying a combustion support substance into the combustionchamber, means for the removal of gaseous combustion products, means forigniting the combustion reaction, an injection device connected to saidcombustion chamber by means of an injection conduit to inject thecombustion support substance into the combustion chamber, a controllerfor controlling injection frequency of the combustion support substanceinto the combustion chamber to thereby control power generated by thecombustion chamber, a conversion chamber within which sub-atmosphericpressure conditions are maintained, wherein the conversion chamberincludes: (i) an elliptical wall having an interior radiation-reflectivesurface in the shape of an ellipsoid defining a focus region forfocusing radiation reflected thereby; and (ii) a planar wall coincidentwith a plane passing through a center of the ellipsoid and perpendicularto a major axis thereof so as that the conversion chamber has asemi-ellipsoidal shape, wherein the planar wall includesenergy-conversion means on an interior surface thereof for convertingradiant energy into electrical energy, and wherein the combustionchamber is enclosed within the conversion chamber and is positioned incorrespondence with the focus region defined by the elliptical wall ofsaid ellipsoid, wherein radiation emitted by the combustion chamber andreflected by the energy conversion means on the interior surface of theplanar wall and radiation emitted by the combustion chamber andreflected by the radiation reflective inner surface of the ellipticalwall are concentrated on the outer surface of the combustion chamber forre-absorption by the external lining thereof.
 2. A system as claimed inclaim 1, wherein said energy conversion means comprise a plurality ofphotovoltaic cells.
 3. A system as claimed in claim 1, wherein theexternal lining has a narrow emission band with a peak in correspondencewith the wavelength at which the energy-conversion means have themaximum conversion efficiency.
 4. A system as claimed in claim 1,wherein the external lining is a material selected from the groupconsisting of micro-structure metal, metallic or dielectric photoniccrystal, oxide or mixture of oxides of rare earths.
 5. A system asclaimed in claim 1, wherein the outer surface of the combustion chamberhas a total area such that the radiant energy emitted by the emissionmeans is equal to the sum of the total thermal energy developed by thecombustion reaction at steady state and of that fraction of radiantenergy that is reflected by the inner walls of the conversion chamber orby the conversion means and reabsorbed by the combustion chamber.
 6. Asystem as claimed in claim 1, wherein said conversion chamber has axeswhose size ranges between 3 and 50 times the diameter of the combustionchamber.
 7. A system as claimed in claim 1, wherein said injectiondevice is a head of the ink-jet type.
 8. A system as claimed in claim 7,wherein said injection head is of the “bubble” ink-jet type.
 9. A systemas claimed in claim 7, wherein said injection head is piezoelectric. 10.A system as claimed in claim 1, wherein the combustion chamber isconstituted by material with high thermal conductivity and able towithstand high temperatures.
 11. A system as claimed in claim 10,wherein part of the inner surface of the combustion chamber is coatedwith a porous layer of material with low thermal conductivity and ableto withstand high temperatures.
 12. A system as claimed in claim 11,wherein the porosities of said porous layer are coated by a catalysingmaterial serving the purpose of lowering the activation temperature ofthe combustion reaction and of limiting the generation of noxiouscombustion products.
 13. A system as claimed in claim 10, wherein thecombustion chamber is made of metallic material.
 14. A system as claimedin claim 13, wherein said metallic material is constituted by tungstenor molybdenum.
 15. A system as claimed in claim 1, wherein saidinjection conduit and said means for supplying the combustion supportsubstance and said means for extracting the combustion gases are made ofa material with low thermal conductivity.
 16. A system as claimed inclaim 15, wherein an that the outermost segment of the exhaust conduitis made of a material with high thermal conductivity to allow combustionproducts to yield the residual heat before exiting the conversionchamber.
 17. A system as claimed in claim 1, wherein the injectionconduit and the means for injecting the combustion support substanceindependently end into the combustion chamber.
 18. A system as claimedin claim 1, wherein the means for the injection of the combustionsupport substance end into the injection conduit before entering thecombustion chamber.
 19. A system as claimed in claim 1, wherein theconversion chamber is formed within a structure made of opticallypolished metallic material.
 20. A system as claimed in claim 1, whereinthe conversion chamber is defined within a structure made of plastic orceramic material and coated with a layer of material with highreflectance.
 21. A system as claimed in claim 2, wherein a surface ofsaid photovoltaic cells facing the interior of said conversion chamberis coated with an optical lining operating on the long wavelengths ofthe electromagnetic radiation as a band pass filter with transmittancepeak in correspondence with the wavelength at which the photovoltaiccells have the maximum conversion efficiency.
 22. A system as claimed inclaim 2, wherein said photovoltaic cells are based on Schottkyjunctions.
 23. A system as claimed in claim 22, wherein said Schottkyjunctions are made of silicon and aluminium.
 24. A system as claimed inclaim 21, wherein said optical lining is made of a material selectedfrom the group comprising: multilayer dielectric lining, metallic liningat the percolation state, metallic photonic crystal, anti-reflectionmicro-structure.
 25. A system as claimed in claim 1, wherein theinjection device is constituted by a miniaturised Bunsen burner.
 26. Asystem as claimed in claim 15, wherein the gaseous fuel injected by saidinjection device belongs to the group comprising: methane, propane,butane, hydrogen, natural gas.
 27. A system as claimed in claim 1,wherein the exhaust conduit is internally coated with catalysingmaterial able to neutralise the noxious products of the combustionreaction.
 28. A system as claimed in claim 1, wherein the exhaustconduit has an articulated path in order to favour the cooling of theexhaust gas.
 29. A system as claimed in claim 1, wherein the injectionconduit has an articulated path in order to prevent the combustionproducts to return towards the injection means.
 30. A system as claimedin claim 1, wherein said ignition means are electrical and thecombustion is started by an electrical discharge, by a spark or by anincandescent filament.
 31. A system as claimed in claim 1, whereinvacuum is obtained inside the conversion chamber.
 32. A system asclaimed in claim 1, wherein inside the conversion chamber is containedan inert gas at sub-atmospheric pressure.
 33. A system as claimed inclaim 1, wherein the conversion chamber is constituted by opticallypolished metallic material.
 34. A system as claimed in claim 31, whereinthe conversion chamber is constituted by optically polished ceramicmaterial.
 35. A system as claimed in claim 1, wherein the inner wall ofthe conversion chamber is coated by a layer having high reflectance overthe whole spectrum of the radiation emitted by the emission means.